This application claims priority under 35 U.S.C. xc2xa7xc2xa7 119 and/or 365 to Appln. No. 9904728-4 filed in Sweden on Dec. 22, 1999; the entire content of which is hereby incorporated by reference.
The present invention relates to a multi-stage unit for processing web packaging material in a food product packaging machine.
Many pourable food products, such as fruit juice, pasteurized or UHT (ultra-high-temperature processed) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example of such a package is the parallelepipedal package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is formed by folding and sealing laminated web packaging material. The laminated packaging material comprises layers of fibrous material, e.g. paper, covered on both sides with thermoplastic material, e.g. polyethylene. In the case of aseptic packages, the side of the packaging material eventually contacting the food product in the package also has a layer of barrier material, such as an aluminium sheet, which in turn is covered with a layer of thermoplastic material.
As is known, such packages are produced on fully automatic packaging machines, in which a continuous tube is formed from the web packaging material.
More particularly, the web of packaging material is sterilized, and then fed to a forming unit where it is longitudinally folded and sealed to form a tube. The tube is then filled with the sterilized or sterile-processed food product, and is sealed by pairs of jaws and then cut at equally spaced transverse bands to form pillow packs, which are subsequently folded mechanically to form the finished, e.g. parallelepipedal, packages.
Upstream from the forming unit, the web material may be fed through a multi-stage processing unit where subsequent auxiliary operations are performed. For example, in case packages are to be made which are provided with opening devices, such as screw caps, hinge caps or pull-tabs, the aforesaid auxiliary operations may include a punching operation for providing holes at selected positions of the web, and the application of the opening devices onto the holes. Opening devices may applied by injection-moulding the opening device directly onto the holes, e.g. as described in WO 98/18609; as an alternative, opening devices may be bonded or thermo-welded to the web packaging material.
In known machines, the web material is step-fed through the multi-stage processing unit by an indexing system including feeding rollers driven by a servomotor which is controlled in response to a position signal generated by an optical sensor detecting a position index on the web, usually a printed pattern such as a bar code repeated along the web at a predetermined pitch.
Particularly in the case of a multi-stage processing unit including a punch station and a moulding station for directly moulding opening devices, extreme position accuracy of the web material in the moulding station is required in order to provide a proper positioning of the web portion surrounding the punched hole inside the moulding cavity for moulding the opening device, and therefore assure correct flow of the injected thermoplastics material into the moulding cavity so as to properly seal the edge of the hole on both sides of the web.
Therefore, a need for further improvement of indexing accuracy exists in the field.
It is an object of the present invention to provide a system for indexing web packaging material in a multistage processing unit of a packaging machine which allows excellent position accuracy in every stage of the unit.
This object is attained by a multi-stage unit for processing a web packaging material in a machine for packaging food products, the unit including at least a first processing station where a first processing operation is performed on the web packaging material, a second processing station where a second processing operation is performed on said web packaging material, and a system for indexing said web packaging material comprising web feeding means for step-feeding said web packaging material through said multi-stage processing unit, sensor means for reading index code means on said web packaging material and a control unit for controlling said web feeding means in response to input signals received from said sensor means, characterised in that said web feeding means includes a first feeding device for step-feeding said web packaging material through said first processing station and a second feeding device for step-feeding said web packaging material through said second processing station, said sensor means including a first sensor detecting a first index code on said web packaging material and generating a first input signal, said first sensor being located in the vicinity of said first processing station, and a second sensor detecting a second index code made on said web packaging material at said first processing station and generating a second input signal, said second sensor being located in the vicinity of said second processing station, said first feeding device and said second feeding device being independently controlled by said control unit in response to said first input signal and second input signal, respectively.